lib: remove a whole bunch of unused time stuff

lib: remove a whole bunch of unused time stuff

QUAGGA_CLK_REALTIME and QUAGGA_CLK_REALTIME_STABILISED aren't used
anywhere in the code.  Remove.  The enum is kept to avoid having to
change the calls everywhere.

Same applies to the workaround code for systems that don't have a
monotonic clock.  None of the systems Quagga works on fall into that
category;  Linux, BSD and Solaris all do clock_gettime, for OSX we have
mach_absolute_time() - that covers everything.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>